1. Field of the Invention
The invention relates to a video signal receiver to which a video signal recording device can be connected.
When receiving video signals, both in the case of cable-conducted transmission and in the case of radio transmission, reception authorization is necessary nowadays for some transmissions. For example, the broadcaster can transmit encrypted transmissions, and only those receiving subscribers who have corresponding decryption means, which can be acquired by licensing, can receive such transmissions. These services are nowadays referred to as Pay TV or Video On Demand. U.S. Pat. No. 4,930,160 describes a video signal receiver in which the display of a video transmission can be blocked by a control device. For this purpose, a transmitted code produced at the transmitter end is evaluated in the control device. Basically, the manufacturers of receiving devices are endeavouring, in the interest of broadcasters, to make it difficult or even impossible to record transmissions subject to access protection.
To date, this has been achieved by influencing the sync signal component of the video signal in the receiving device in such a way that display on a cathode ray tube device is readily possible, but recording with a video recorder produces extremely poor quality, or is even impossible. In the case of a known solution, in a color coder which converts digitally transmitted and received video signals into analog signals for driving conventional television sets and video recorders, the sync signal component is provided with deliberately introduced phase jitter. The synchronization devices of the television set, which normally have a relatively small time constant, can follow this phase jitter and produce a high-quality picture. Recording devices, on the other hand, in particular video recorders, cannot follow the phase jitter of the sync signal component on account of the inertial mass of the rotating video drum. Consequently, such transmissions can only be displayed on a cathode ray tube device; they cannot, however, be recorded. The receiving device terminals to which recording devices and cathode ray tube devices can be connected are not differentiated. However, the deliberate insertion of interference in the sync signal component can lead to signal processing problems in some television sets.
In addition, circuits for time-base correction already exist, and they can be used to correct the video signals which have been subjected to interference, with the result that the the video signals are suitable for recording all the same.
A device has become known heretofore from European patent publication EP-A-0 267 039 wherein it is possible to receive a video signal with an identifier that indicates whether or not recording of the video signal is allowed. If recording is not allowed, then the video signal is influenced by the recording device in such a way that recording thereof at a reasonably acceptable quality is no longer possible.
European publication EP-A-0 348 218 describes a double-deck video recorder. Any recording operation is blocked if the video signal to be recorded contains a copy-protect signal.